The present invention relates to a structured catalyst for the removal of nitrogen oxides from exhaust gases from internal combustion engines operated predominantly at a lean air/fuel ratio by selective catalytic reduction using ammonia or a compound which can be decomposed into ammonia as reducing agent. Such internal combustion engines are diesel engines and directly injected petrol engines. They are referred to collectively as lean-burn engines.
The exhaust gas from lean-burn engines contains not only the usual pollutant gases carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx but also a relatively high proportion of oxygen of up to 15% by volume. Carbon monoxide and hydrocarbons can easily be made nonpolluting by oxidation. The reduction of the nitrogen oxides to nitrogen is significantly more difficult because of the high oxygen content.
A known method of removing nitrogen oxides from exhaust gases in the presence of oxygen is the process of selective catalytic reduction (SCR process) by means of ammonia which can also be generated in situ from a precursor compound such as urea. In this process, comproportionation of the nitrogen oxides with ammonia takes place with formation of nitrogen over a suitable catalyst, referred to as SCR catalyst for short.
Since internal combustion engines are operated in transient driving cycles in the motor vehicle, the SCR catalyst has to ensure very high nitrogen oxide conversions at good selectivity even under widely varying operating conditions. Both complete and selective nitrogen oxide conversion at low temperatures and selective and complete conversion of high concentrations of nitrogen oxide as occur, for example, during full-load driving in very hot exhaust gas have to be ensured. In addition, the widely varying operating conditions present difficulties in the exact metering of ammonia, which should ideally be introduced in a stoichiometric ratio to the nitrogen oxides to be reduced. As a result, severe demands are made on the robustness of the SCR catalyst, i.e. its ability to reduce nitrogen oxides to nitrogen with high conversions and selectivities over a broad temperature window at highly variable space velocities over the catalyst and a fluctuating supply of reducing agent.
EP 0 385 164 B1 describes all-active catalysts for the selective reduction of nitrogen oxides by means of ammonia, which contain titanium oxide and at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum and cerium together with an additional component selected from the group of oxides of vanadium, niobium, molybdenum, iron and copper.
U.S. Pat. No. 4,961,917 claims catalyst formulations for the reduction of nitrogen oxides by means of ammonia, which contain zeolites having a silica:alumina ratio of at least 10 and a pore structure which is linked in three dimensions by pores having an average kinetic pore diameter of at least 7 Angstrom together with iron and/or copper as promoters. EP 1 495 804 and U.S. Pat. No. 6,914,026 disclose methods of improving the stability of such zeolite-based systems under hydrothermal aging conditions.
The SCR catalyst formulations described in the documents mentioned, which represent the present prior art, all display good nitrogen oxide conversions only above 350° C. In general, the reaction proceeds optimally only in a relatively narrow temperature range. This conversion optimum is typical of SCR catalysts and is due to the mode of operation of the catalysts.
As a result of the optimal stoichiometry of the reaction, the reduction of a 1:1 molar mixture of nitrogen monoxide NO and nitrogen dioxide NO2 with ammonia NH3 proceeds many times as quickly as the reduction of pure nitrogen monoxide NO. The nitrogen oxides NOx present in the exhaust gas from lean-burn engines comprise predominantly NO and have only small proportions of NO2. However, since the oxidation of NO to NO2 has to be promoted kinetically by an oxidation catalyst at temperatures below 300° C., SCR catalysts do not display any significant conversions in the low-temperature range if they do not have a certain oxidizing power. On the other hand, an excessively high oxidizing power at temperatures above 350° C. leads to ammonia being oxidized by the high oxygen content of the exhaust gas from lean-burn engines to form lower-valent nitrogen oxides such as nitrous oxide N2O. This firstly results in loss of the reducing agent required for the SCR reaction and, secondly, NOx in the form of the undesirable secondary emission N2O is formed. This leads overall to a significant limitation of the operating window of low-temperature SCR catalysts to a very narrow temperature range. For example, SCR catalysts containing noble metals display very high NOx conversions in the range from 100 to 250° C., but the temperature range in which the catalyst operates with satisfactory selectivity to nitrogen is generally restricted to from 20 to 50° C.
The conflict in terms of objectives between an oxidizing power which is too high and consequently a lack of selectivity and an oxidizing power which is too low and therefore unsatisfactory lower-temperature activity is the reason why SCR catalysts such as the formulations mentioned in EP 0 385 164 B1 or U.S. Pat. No. 4,961,917 have to be used either in combination with an upstream oxidation catalyst or/and in combination with a further catalyst capable of reducing nitrogen oxides for removing nitrogen oxides from the exhaust gas from lean-burn engines in order to be able to ensure removal of the nitrogen oxides at all operating temperatures which occur during driving operation, which are in the range from 200° C. and 600° C. The supplementary catalyst capable of reducing nitrogen oxides can be a low-temperature SCR catalyst, a nitrogen oxide storage catalyst, an HC-DeNOx catalyst or another suitable, reduction-active catalyst technology or combinations thereof.
For example US 2006/0039843 discloses such a system solution. Paragraph [0062] describes, as advantageous embodiment, a system for purifying exhaust gas, in which a substrate coated with an SCR catalyst is arranged between the reducing agent injector and a catalyst support which is coated with an SCR catalyst and an ammonia decomposition catalyst. The SCR catalyst formulations are, in a preferred embodiment according to this text, selected so that the first catalyst operates optimally at relatively high operating temperatures while the second catalyst is more suitable for use in cooler segments of the exhaust gas system.
DE 103 60 955 A1 describes an exhaust gas purification unit for an internal combustion engine, in which ammonia utilized as reducing agent in the SCR reaction is generated from appropriate exhaust gas constituents over a first (in the flow direction) catalyst when a rich exhaust gas composition is present. The ammonia generated by the first catalyst is stored temporarily on a second (in the flow direction) catalyst in the case of a rich exhaust gas composition. In the case of a lean exhaust gas composition, the nitrogen oxides present in the exhaust gas are reduced using the temporarily stored ammonia. Downstream of the second catalyst, there is a third, noble metal-containing catalyst which comprises at least one of the platinum group metals Pt, Pd or Rh on support materials which are able to store ammonia in the case of a rich exhaust gas composition and liberate ammonia in the case of a lean exhaust gas composition. According to this document, the temperature activity ranges of the standard SCR catalyst used in the second position and the noble metal-containing catalyst complement one another so that the proposed exhaust gas purification unit is able to increase the nitrogen oxide conversion considerably, especially at low temperatures.
Although such system solutions ensure that the nitrogen oxides present in the exhaust gas from the lean-burn engine are largely removed in transient operation of the engine, they have considerable disadvantages. Thus, space has to be provided in the vehicle for installation of all catalysts required. Furthermore, each catalyst generates a measurable exhaust gas counterpressure which leads to reductions in the engine power available for operating the vehicle and thus ultimately to increased fuel consumption. In addition, such system solutions require complicated studies on the vehicle application during the development phase of a vehicle so as to ensure that all catalysts are arranged in an optimal position with regard to conversion and selectivity behavior. Here, the optimal position of the catalysts is critically dependent on the achievable operating temperatures and thus firstly on the distance from the engine and secondly on the heat losses in the exhaust gas unit. Of course, each further component incurs higher costs.
In paragraph [0021] of the abovementioned document DE 103 60 955, it is proposed that, in order to minimize the pressure drop, the third, noble-metal containing catalyst be applied to an outflow-end zone of the second catalyst, with this zone covering from 5 to 50% of the total length L of the second catalyst.
The proposed solution reduces the difficulties caused by the configuration as a system to only a limited extent. Since the third catalyst configured as an outflow-end zone of the second catalyst is a noble metal-containing catalyst and, as a result of its high oxidizing power, displays unsatisfactory selectivity to nitrogen at temperatures above 250° C., high N2O secondary emissions are to be expected above this temperature.